The products of the RAG-1 and RAG-2 genes are known to be the only lymphoid-specific factors required for the rearrangement of antigen receptor genes (generally known as V(D)J recombination). We have made a large number of mutations in the mouse RAG-1 and RAG-2 genes, for the purpose of understanding the functions of different parts of the proteins. So far, we have learned that large parts of both proteins are dispensable for recombination activity. The N-terminal 35% of RAG-1 can be removed without a significant decrease of activity; similarly, the C- terminal 25% of RAG-2 is not essential. The deleted regions include a zinc-finger motif in RAG-1 and a highly acidic sequence in RAG-2, which are thus shown to be non-essential.